Why does my external hard drive only show 2.2TB?
Older hard-drive enclosures may be limited in the size of drive they can present to your system.
External hard drives are exceptionally useful for expanding storage capabilities for both backups and data management. While external hard drives are often sold in preconfigured packages by manufacturers, another popular option is to purchase an external hard-drive enclosure and then use any drive of your choice in it. This is beneficial because as your demands for storage increase, you can replace the enclosure's drive with a larger one.
These days, the availability of hard drives with 4TB of storage are enticing for people to swap into their existing enclosures; however, when doing so they may find that the system will only recognize 2.2TB of the drive, regardless of how they partition or format the device.
While modern file-system formats such as HFS+, NTFS, and ExFAT ought to handle volume sizes of between hundreds of gigabytes to zettabytes, and though operating systems like OS X have increased the maximum volume size from 2TB to 8EB (exabytes) over the years, there are hardware limitations that may limit the size of the volume that can be used. If you are using an older drive enclosure for your large hard drive, then the controllers on it may not be capable of handling over 2.2TB, regardless of the software environment being used around it.
This problem happens because of the use of LBA (logical block addressing) in modern hard-drive controllers coupled with a hardware-based limit of how many blocks can be included in the LBA scheme. Early LBA controllers used 32-bit (or lower) addressing coupled with a maximum supported block size of 512 bytes. This means they support up to 2^32 or 4,294,967,296 blocks for a device, and with each block being 512 bytes, this translates to a maximum of 2,199,023,255,552 bytes, or 2.199TB.
Unfortunately in many cases these limits are hard-coded in enclosure's firmware, so even though modern drives use 4,096-byte sectors, the system will still only address these sectors as 512 bytes in size, resulting in both a waste of space and degraded performance.
The only way around this problem is to replace your drive enclosure with a new one that has proper support for both 48-bit (or greater) LBA and 4,096-byte sectors in hard drives. Luckily most enclosures on the market today do support this, so if you run into this problem, you should be up and running in no time.